APPENDIX side-chains where only one side-chain is to be converted into carboxyl on account of their less energetic action. The oxidation of a halogen-substituted side-chain by the usual oxidising agents is much more readily accomplished than that of a simple alkyl group. A similar case is that of naphthalene telrachloride, C10HiSCl4, which, though an additive compound, is much more readily converted into phthalic acid than naphthalene itself. PREPARATION 91. m-Nitro-, m-Amino-, m-Hydroxy-beiizoic Acids.— This series of compounds merely furnishes an exercise in the processes previously described and illustrates the application of the same reactions in the case of a substituted benzene derivative containing a nitro-groiip. It also illustrates the manner in which meta-compounds of benzoic acid may be indirectly prepared where a direct method is inapplicable. PREPARATION 93. Benzoin.—As a small quantity of potassium cyanide is capable of converting a large quantity of benzaldehydeTinto benzoin, the action of the cyanide has been explained as follows : The potassium cyanide first reacts with the aldehyde and forms a cyanhydrin, which then condenses with another molecule of aldehyde, hydrogen cyanide being finally eliminated (Lap worth), /OH /OH CUIISCH< + QH8CHO == QH5.Cf------CH(OH)C6Ha. \CN XCN = QH5.CO.CH(OH).QH5 + HCN. The same reaction occurs with other aromatic aldehydes (anisaldehyde, cuminol, furfurol, &c.)« Benzoin yields hydrobelizbin on reduction with sodium amalgam, and desoxybenzoin, C(!Hr>CO.CH.,.C(iH5, when reduced with zinc and hydrochloric acid. The latter, which contains the group CO.CH.,.C(iH-, behaves like malonic ester, the hydrogen of the methylene group being- replaceable by sodium, and hence by alkyl groups.